The present invention relates to a writable/readable optical recording medium, and more particularly to an improvement in reflectivity of an optical recording medium which has either one or both of a recording layer and a light reflective layer on an optically transparent substrate.
It is generally well known that organic dyes such as cyanine or phthalocyanine dyes are used in a recording film of a so-called "writable" or "rewritable" optical recording medium.
In a method for writing information signals on such a rewritable optical recording medium, a laser beam is converged onto a fine area of a recording film and is adapted to be converted into a thermal energy to thereby change characteristics of the recording film (i.e., to form pits). In order to perform a smooth change of the characteristics of the recording film, it is general to take a so-called air-sandwich structure such that two pieces of substrates on which recording films are formed are provided for a recording medium and the substrates are coupled together with their recording medium facing each other.
A "write" laser beam used for such a type of the rewritable optical recording medium is projected from a transparent substrate side so that optically readable pits are formed in the recording films. The "read" laser beam for reproducing recorded information data has a smaller output power than that of "write" laser beam, and a contrast between an area where the pits are formed and another area where no pits are formed is read out as an electric signal.
On the other hand, there is a so-called ROM (Read Only Memory) type medium in which information data have been recorded in advance unlike the above-described recording medium. This type recording medium has been widely used in fields of audio signal recordation and information data processor. In this case, prepits corresponding to data to be reproduced have been formed on plastic substrates through a press process in advance, reflective layers made of metals such as Au, Ag, Cu and Al are formed on the substrate, and further protective layers are formed thereon. A typical medium of this ROM type is a compact disc which is generally referred to as CD. A specification of recording and reading signals for CDs is standardized. A compact disc player (CD player) has been widely used as a CD playback system essentially in accordance with the standards.
By the way, the writable or rewritable recording medium is common with the CDs in using laser beams and also in shape of medium, i.e., disc shape. In view of this fact, a writable medium which conforms with the CD standards and is capable of being used in CD players without any change has been vigorously developed.
Conventionally, in order to obtain a high reflectivity in conformity with the CD standards, for example, the materials for the recording films and the reflective films are changed or modified, or discrete enhance films are interposed between the recording and reflective films. However, there is a limitation against the enhancement of the reflectivity by selection of the reflective film material, and it would be very difficult to change the recording films per se. Moreover, in the case where the enhance film is used, although it would be possible to enhance the reflectivity to some extent, this causes a tracking error and an error rate to deteriorate.
On the other side, another attempt has been made. For example, an anti-reflective film made of inorganic compounds such as magnesium fluoride and cryolite is provided on one side of the substrate to reduce a light quantity loss as much as possible and to perform a high efficiency recordation and playback.
However, since such an attempt to form the anti-reflective films requires large scale equipment such as a vacuum system for depositing or sputtering, it would not be easy to form the anti-reflective films. It is general that resins such as PMMA, PC and the like are used for the substrates from a productive point of view. In this case, if the substrates are left under a high temperature circumstance or a circumstance where a heat change is remarkable, since there is a remarkable difference in expansion coefficient between the substrates and the anti-reflective films, a strain would be generated to cause a crack or a film separation. It would be therefore impossible to enjoy the advantage of the uniform reflectivity with such anti-reflective films.